There e expectations for applications of superconductors in, for example, power transmission cables, fusion reactors, magnetically levitated trains, particle accelerators, magnetic diagnostic devices (MRI), etc. It is desirable to increase the critical current density of the superconductor. Thereby, a large current can be caused to flow through the same cross-sectional area. For example, it is desirable to obtain a high critical current density in a magnetic field.
Superconduction is the phenomenon in which the resistance value is completely zero, discovered using mercury by Kamerring Onnes of the Netherlands who developed a refrigerating machine. Subsequently, the superconducting dislocation temperature (Tc) of the upper limit according to the BCS theory was thought to be 39 K. This was the Tc of a type-I superconductor. A type-II superconductor that was discovered by Bednorz et al. in 1986 had results exceeding 39 K. This led to the development of an oxide superconductor usable at the temperature of liquid nitrogen. The oxide superconductor is a type-II superconductor in which it is possible for superconducting and non-superconducting states to coexist. Today, many high temperature oxide superconductors that are usable at the temperature of liquid nitrogen are sold in lots that are 500 m long. There are expectations for the application of superconducting materials to various large-scale devices such as superconducting power transmission cables, fusion reactors, magnetically, levitated trains, particle accelerators, magnetic diagnostic devices (MRI), etc.
A bismuth-based superconducting wire which is called the first generation and an yttrium-based superconducting wire which is called the second generation are the major superconducting wires developed as high temperature oxide superconductors. For the first generation in which not less than 60 vol % of silver is used, manufacturers have successively withdrawn; and there are few companies in the world currently manufacturing. On the other hand, for the second generation in which the base member is inexpensive and the physical strength is superior, the total length of strand sold has exceeded 3,000 km. A 50 MVA direct current power transmission cable system made using a large amount of wire already has been in actual operation for more than two years as of August of 2014. A direct current power transmission cable system having a capacity of 500 MVA has been operating from September of 2014. The power transmission capacity of 500 MVA is large-scale electrical power corresponding to nearly 50% of the electrical power of a standard nuclear reactor.
Cumulatively, more than 3,000 km of wire has been sold. All of more than 20 km of strand length actually delivered and actually applied is made by TFA-MOD (Metal Organic Deposition using TriFluoroAcetates). TFA-MOD is the first method that has been actually applied and can stably manufacture 500 m long wire and supply large amounts [1]. Other major manufacturing methods of the second generation include Pulsed Laser Deposition and Metal Organic Chemical Vapor Deposition. The composition control of these methods is problematic; and stable mass production of 500 m long wire is not being performed at the current point in time. Therefore, at the current point in time, the wire market share of TFA-MOD is substantially 100%.
This fact does not negate the future of Pulsed Laser Deposition or Metal Organic Chemical Vapor Deposition. Mass production by physical vapor deposition in which composition control is difficult would be possible if technology is developed so that three types of elements can be projected through a vacuum, have atomic weights differing by a factor of 2 or more, and can be controlled by an inexpensive method to have a composition shift like TFA-MOD of 1% or less. This problem has been unresolved for more than 28 years since 1987.
On the other hand, wires by Pulsed Laser Deposition or Metal Organic Chemical Vapor Deposition are in the lead for coil applications in which high magnetic field characteristics are necessary. Because artificial pins for improving the magnetic field characteristics are easily introduced the wires by these methods are one step ahead for the magnetic field characteristics. As described above, the composition control for these methods is difficult; and there is no report of mass production or implementation of a wire on the order of 500 m by these methods.
On the other hand, TFA-MOD which is being used in mass production, had problems in the past. If no countermeasures are performed in TFA-MOD, the maximum thickness of a superconducting film obtained by one film formation is about 0.30 μm. In this method, organic substances having a density of about 2 g/cm3 change to metal oxides of 3 to 4 g/cm3 in the decomposition reaction in the pre-bake; and the molecular weight (the formula weight) 3 g decreases drastically. Therefore, the volumetric reduction rate is about 85%, Accordingly, cracks occur easily due to a large drying stress. The critical film thickness in the highly purified solution is about 0.30 μm. Because a large current value is necessary in the superconducting cable, a film thickness of about 1 μm is necessary. Therefore, for TFA-MOD, increasing the film thickness has been attempted by repeated coating and the like.
Increasing the film thickness by repeated coating used in conventional MOD was difficult to apply to TFA-MOD. The oxides that are formed after the pre-bake contact trifluoroacetic acid which is a strong acid having a pH near 0 in the next film formation. A heterogeneous interface is formed by the reaction of the strong acid and the oxides. The characteristics degrade from the heterogeneous interface as the starting point. If the yield is 95% for 10 m wire, in the case where a 500 m long, wire is manufactured by three coatings such technology, a good part would be obtained only once every 200 times. Technology for a thicker one-coating film thickness [2] necessary for TFA-MOD.
The crack preventing agent used for one coating having a thicker film thickness is an organic substance. It was considered that many crack preventing agents would exist among the more than a million types of organic substances. However, from recent research, it is known that there are only two systems and about ten types of film thickness-increasing materials that are applicable to TFA-MOD. For a wire made by technology for a thicker one-coating film thickness, an unstable interface such as that recited above does not exist; and the yield improves drastically. It is considered that this technology is one driving force of the rapid progress of TFA-MOD that has attained substantially 100% of the market share.
The fields of application of superconducting wires is broadly divided into power transmission cable applications which are used with magnetic fields of substantially zero, and coil applications which are used under a strong magnetic field. Applications of wires made by TFA-MOD which is the first process that made it possible to supply large amounts of wire mainly are for power transmission cables at the current point in time. An artificial pin is necessary to use the wire made by TFA-MOD in a magnetic field. In TFA-MOD that makes the superconductor from a homogeneous solution [3], the size of the artificial pin easily becomes larger than those of other methods. Therefore, the magnetic field characteristics did not improver. The wires made by TFA-MOD could not be utilized in a magnetic field.
The artificial pin that improves the magnetic field characteristics is, for example, a non-superconducting region formed inside the superconductor. The coexistence of the non-superconducting region is possible in a type-II superconductor. If the quantum flux lines and the strong magnetic field created by the superconduction are trapped by the artificial pin. The other portions stably function as a superconductor; and the characteristics in the magnetic field are maintained. The ease of the trapping of the quantum flux line by the artificial pin has a relationship with the size of the artificial pin.
The artificial pin is a non-superconducting portion. A force (a pinning force) that pushes back the flux is generated at the interface between the artificial pin and the superconducting portion. It is considered that the trapping by the pinning force is realized to the utmost as the size of the artificial pin approaches the size at which one quantum flux exists. When the size of the artificial pin becomes large and multiple quantum flux exist inside the artificial pin, the Lorentz force acts on each of the multiple quantum flux and pushes the adjacent quantum flux. Therefore, in such a case, the quantum flux easily crosses the interface and enters the superconducting portion. It is considered that the superconducting characteristics degrade due to the energy loss caused thereby. The artificial pin size becomes large in the heat treatment in almost all reports to date.
The increase of the size of the artificial pin is disadvantageous also from the aspect of the number of artificial pins or the density of the artificial pins. It is assumed that the artificial pin has a particle configuration; and it is assumed that the artificial pin obtained has a radius that is twice the target radius. The volume of each of the multiple artificial pins is 8 times the assumed size; and the density of the artificial pins per introduced substance amount is ⅛. In the case where an artificial pin having a size of 3 nm is attempted but artificial pin having a size of 30 nm is obtained, the volume of the obtained artificial pin is 1000 times the attempted size. The number of artificial pins obtained becomes 1/1000 of the number of artificial pins attempted. This is the actual state of the artificial pins formed by TFA-MOD in which the homogeneity is superior but the improvement of the magnetic field characteristics is difficult.
The optimal size of the artificial pin may be shifted from 3 nm due to the temperature dependence or the magnetic field dependence. There have been no actual results or measurement results in the past for the formation of a homogeneous artificial pin having a size of 3 nm. Therefore, it is unclear whether or not the size of 3 nm is most effective.
In an attempt to form an artificial pin having a 3 nm size, it has been attempted to form an artificial pin of BaZrO3 and the like by PLD and MOCVD which are physical vapor deposition. It is considered that the artificial pins that can be made by PLD are broadly divided into two types. One is an artificial pin for which particle growth and the like are performed independently without effecting the perovskite structure. The other is an artificial pin having a correlation with the perovskite structure.
For the type in which the artificial pin is formed as foreign matter without having a correlation with the perovskite stricture, the film formation temperature is a high temperature of 700 degrees C. or more. Therefore, the separation between the artificial pin and the superconductor having the perovskite structure becomes severe. It is considered that the reason there are substantially no reports of this type is because the artificial pin size is 100 nm to 1 μm and there is no effect for both size and amount.
On the other hand, for PLD, there are substances that form an artificial pin while having a correlation with the perovskite structure. Such a substance is for example, BaZrO3. The size of the artificial pin can be relatively small to form the artificial pin while having the correlation. While an observation image having a minimum of 6 nm has been reported, it is considered that the average is 10 nm or more. However, because this type of substance has a correlation with the YBCO layer, this type of substance affects the YBCO layer and causes a decrease of the number of oxygen atoms and degradation of the superconducting characteristics of the superconductor. The decrease of the critical temperature (Tc) is caused to occur, which can be said to be crucial to the superconductor. For a heterogeneous wire in which the critical temperature Tc decreases by location, the coil design and the implementation is difficult.
There is also report that Tc substantially does not, decrease in the case where the BaZrO3 amount is extremely low. In the report, the Tc measurement is performed using a faint current. For a wire that has a cause of instability in the internal structure of the wire, nonuniformity occurs when the current value is increased; and the characteristic degradation becomes large. It is considered that this is one reason that the application to coils does not progress for wires that are obtained by physical vapor deposition and have unstable lengths.
Generally, heating is performed to not less than 700 degrees C. when the superconducting cable is a film in the film formation by physical vapor deposition such as PLD, MOCVD etc. There is no effect on the magnetic field characteristic improvement as recited above when forming an artificial pin that does not affect the perovskite structure of the superconductor. If an artificial pin that affects the perovskite structure is formed, the Tc decreases; and nonuniformity of the interior occurs. It is considered that the size of the artificial pin has an average of about 10 nm and minimum of about 6 nm, which is not a size that has an effect on the magnetic field characteristic improvement. Therefore, it is considered that new technology not existing in the past is necessary for the mass production of a practical wire by physical vapor deposition such as PLD, MOCVD, etc.
On the other hand, even for TFA-MOD which has substantially 100% of the market share of cable applications, the realization of the artificial pin was still far from certain. A Tc of 90.7 K is maintained if an artificial pin of Dy2O3 is formed. This is because Dy2O3 grows separately from YBCO. The size of the artificial pin is 20 nm to 30 nm; and there are substantially no effects.
On the other hand, in the case where a substance that affects the perovskite structure is introduced, the effects on the internal structure made by TFA-MOD started from the homogeneous gel film are extremely large. Therefore, the perovskite structure itself is not formed and a non-superconductor is formed. The superconductor made by TFA-MOD is, affected by impurities, etc., more easily than is physical vapor deposition. By TFA-MOD, to begin with, the formation of an artificial pin of BaZrO3, etc., is difficult. Even in the case where the artificial pin is formed, the Tc decreases or the heterogeneity occurs more strongly.
In the Y-based superconducting wire as described above, it is necessary to improve the Jc-B characteristic by introducing the artificial pin while maintaining Tc. However, all of the sizes of the artificial pins become large for the artificial pins when the perovskite structure and Tc are maintained. In a system that affects the perovskite structure, there are various negative effects; and it is not practical. Therefore, a completely new method is desirable for a superconductor in which the perovskite structure is maintained and only a portion of the superconductor functions as the artificial pin.
Currently, two methods are being developed as solutions. An application has been filed for part of these. Both are based on an artificial pin having a unit cell size. Both are atomic substitution-type artificial pins that become artificial pins by atomic substitution. This is called ARP (Atom-replaced Pinning). In one, a SmBCO unit cell is dispersed in the YBCO perovskite structure; and substitution is performed for Ba and Sm of the SmBCO. In this method, the unit cell due to the so-called Ba substitution is utilized as the non-superconductor.
In the other method, a PrBCO unit cell is formed in the YBCO perovskite structure. While it is widely known that PrBCO is non-superconducting, the principle of becoming non-superconducting is unclear. In the current state, the principle of Pr becoming non-superconducting is temporarily considered to be as follows. It is considered that PrBCO is trivalent at the vicinity of 800 degrees C. to form the perovskite structure. A normal perovskite structure is formed. However, when cooled, it is considered to be an intermediate body between trivalent and tetravalent. In such a case, the number of oxygen atoms between two unit cells increases; and the c-axis length becomes short. The XRD measurement results affirm this hypothesis. The increase of the number of oxygen atoms causes degradation of the superconducting characteristics. In the case of Pr, while the detailed mechanism is unclear, it is highly likely that the PrBCO completely becomes non-superconducting and four adjacent unit cells in the a/b plane are caused to become non-superconducting.
If the process recited above is correct, characteristic degradation of about 5 times the introduced Pr amount occurs. Moreover, the degradation of the characteristics of 5 times has been confirmed many times from experimental results. However, even in the case where, in the width direction, the size is only three unit cells, the artificial pin has a size of 1.2 nm. While there is no past example of such a small size of the artificial pin, this is too small for the target of 3 nm; and the effect is small.
A superconductor obtained by causing PrBCO and YBCO to coexist is in an extreme dispersion state. In the extreme dispersion state, the PrBCO is dispersed in solitary unit cells. It is known from experimental results that the SmBCO unit cells formed by mixing SmBCO with YBCO also have extreme dispersion. If Ba and Sm are substituted for the SmBCO unit cell, the unit cell as an entirety can be a non-superconducting artificial pin. Further, if the adjacent unit cells are affected, a unit cell of a maximum of about 5 times may become an artificial pin. However, even in such a case, the size of the artificial pin is only 1.2 nm.
By forming the artificial pin at the atomic level by either method, the size is too small. By aggregating these artificial pins, the size can be in the vicinity of 3 nm; and therefore, a higher critical current density in a magnetic field can be expected.